A variety of devices utilize a rise in temperature to alter an element to indicate when temperature exceeds a certain level. Several devices include elements which decompose, melt, boil or sublime at predetermined temperatures. Some devices use a normally opaque material to reveal a previously concealed message. One such material is benzil, which when heated becomes transparent.
In many applications the detection of a simple rise in temperature is not sufficient. A large number of pharmaceuticals and foods expire as a function of both time and temperature. Composite polymeric structures must be maintained at a set temperature for a set time to achieve proper cure. Effective indicators of expiration, shelf life, or cure must therefore reflect time as well as temperature. Disappearance of an ablative material is used in some shelf-life indicators to gradually reveal an underlying message. Others use advancing color fronts generated by passage of liquids or gases through wicks.
Some inventory control systems chronologically predict when a product may expire. Numbers or color-coding indicate the day of the month or other calendar period when an observer is to dispose of the product without considering the actual effect of ambient temperature.
Devices which dispense volatile materials indicate remaining capacity in several ways. The overall dispensing structure may be transparent to allow direct observation of the contents. If the volatile material contains dye, the color may become more intense as the liquid evaporates and the concentration of the remaining dye increases. Alternatively, the dye may gradually disappear from the surface of the dispenser as the dye concentrates in other regions or precipitates out of solution.